Telecommunications systems are generally structured with an architecture that complies with the layer organization of the open system interconnection (OSI) communications model as standardized by the International Organization for Standardization (ISO).
The OSI communications model defines the management of a data transmission service by means of seven superposed protocol layers: the physical layer (layer 1); the data link layer (layer 2); the network layer (layer 3); the transport layer (layer 4); the session layer (layer 5); the presentation layer (layer 6); and the application layer (layer 7).
The first three layers 1, 2, and 3 are referred to as “low” or “media” layers and they relate to setting up the connection and transporting the data. The following four layers are referred to as “high” or “host” layers and they are responsible for processing data. This organization thus enables the telecommunications system to implement the service associated with the processed data.
The data link protocols satisfy service requests coming from the network layer and they perform their function by sending service requests to the physical layer.
Exchanges of signaling between two communications entities that are joined together by a transmission channel are controlled at data link layer level by means of a frame structure known as medium access control (MAC). With reference to FIG. 1, MAC frames are encapsulated in a frame structure (known as “physical” frames) by the physical layer PHY, prior to being transmitted over the transmission channel.
FIG. 2 shows an example of how a physical frame (PPDU structure) is made up for the single-frame MAC mode of the ECMA-368 standard as defined in the document “High rate ultra-wideband (UWB) PHY and MAC standard”, 3rd edition, December 2008. Layer 1, referred to as the PHY layer, is made up of two protocol sublayers: a physical layer convergence protocol (PLCP) layer and a physical medium dependent (PMD) layer. The PLCP layer provides frame synchronization and allocates one or more logic channels (MAP service data unit (MSDU)) coming from the MAC layer at PHY frame level. The PMD layer generates the data field (frame payload). These two layers give rise to a transmission unit, referred to as a PLCP protocol data unit (PPDU), transmitted over the PHY layer. The PPDU has a PLCP preamble, a PLCP header, and a PSDU.
Nowadays, telecommunications systems are seeking flexibility in the PHY/MAC transmission mechanism for the purpose of delivering a given data rate D at a transmitter-to-receiver distance d with a guaranteed quality of service (QoS), which is described at the level of the PHY layer by an optimum binary error rate (BER) referred to as a target BER (BERt). These flexible systems comprise one or more transmission interfaces.
A transmission interface includes the physical layer PHY that encompasses one or more transmission modes and transmission protocols specific thereto (MAC) in order to implement these transmission modes.
The term “transmission mode” is used below to designate a transmission technique (orthogonal frequency division multiplexing (OFDM), multiple-input multiple-output (MIMO) technique (spatial mapping, spatial division multiplex, etc.), spreading, etc.), associated with an error-correcting code scheme (signal binary coding (SBC)) and digital modulation, referred to as a modulation and coding scheme (MCS) which is typically 16-QAM 1/3, 64-QAM 3/4, etc., and also with a transmission bandwidth Bw and with a transmission carrier frequency (optical, RF, etc., possibly having a value of zero) enabling the signal to be generated in a spectrum band that is dedicated to transmitting the signal (base band, radio band, infrared band, optical band) and delivering a data rate D.
The data rate D is calculated on the data field, usually referred to as the data payload or the frame payload, that is incorporated in a PSDU, and it does not take account of the frame format at MAC layer level.
Thus, in order to deliver a certain data rate D with a quality of service QoS over a distance d, it is possible, in a so-called “flexible” telecommunications system, to select one transmission mode from a plurality for a given communications unit.
A communications entity may equally well be a mobile terminal or a fixed terminal or any type of access point to an access network.
Selection is thus based on the transmission mode that is best suited for guaranteeing a data rate D and a QoS over a distance d.
The preamble of the PPDU is dedicated to frame synchronization and to other functions. It is made up of a sequence for synchronizing the two communicating entities that seek to communicate and it is specific to the protocol of the MAC layer. For a system of the ECMA-368 type, known as “Wi-Media/ECMA-368”, the preamble also includes OFDM symbols dedicated to estimating the propagation channel.
The PHY header or the PLCP header of the PPDU gives the characteristics of the associated transmission mode to the transmission interface j under consideration, information relating to the size of the frame payload data field in the PSDU, the MAC transmission mode (whether or not the data field is fragmented into subframes (burst mode)), the sequencing of the subframes, information associated with the link (transmitter-to-receiver distance, radiated power, etc.), and other characteristics. This transmitted data is protected using codes (Reed Solomon code for an ECMA-368 system). Tail bits and pad bits are used to reinitialize the coder specified in the PLCP header.
The PSDU contains, in particular, the information that is to be transmitted (frame payload), which information is associated with one or more transmission modes of the transmission interface j. The PSDU corresponds to the data field and may be made up of one or more MSDUs or it may be a fragmentation thereof.
For a PSDU made up of an MSDU, the PSDU contains, in particular, the information for transmitting at the data rate D for a given transmission mode, and the MAC protocol data unit (MPDU) protocol at the level of the MAC layer controls synchronization of the transmission of the PHY frame formed of an MSDU.
For a PSDU made up of a plurality of MSDUs, the MSDUs are fragmented in subframes. Under such circumstances, the MAC transmission mode is said to be burst mode. The MAC layer controls only the first or “synchronization” frame, the other frames are controlled at PHY layer level. Additional fields are added both in the PLCP header and in the PSDU in order to manage this fragmentation of the MSDUs and how they are distributed over a plurality of subframes. These various MSDUs may comprise a plurality of transmission modes of the transmission interface j. The frame that is built up is referred to as a superframe.
The PSDU may be associated with a frame check sequence (FCS) and by bits known as tail bits for reinitializing the channel coder (associated with the transmission mode). Pad bits ensure that the frame is of a size that is compatible with the MAC frame format.
The following examples of communications entities may be said to be “flexible”; they have one or more transmission interfaces respectively associated with one or more transmission modes.
The first example relates to a communications entity that has transmission interfaces compatible respectively with a MIMO IEEE802.11n system, with a SISO UWB-OFDM system (Wi-Media/ECMA-368), and with a UWB-OFDM system at 60 gigahertz (GHz) with transmission modes that make it possible to obtain the same target data rate, given in this example to within about 3%, associated with an upper limit of about 8% in maximum value, making it possible to select various transmission modes.
A MIMO IEEE802.11n system as defined in the document “IEEE P802.11n™/D4.01 draft standard for information technology telecommunications and information exchange between systems—local and metropolitan area networks—specific requirements—Part 11: wireless LAN medium access control (MAC) and physical layer (PHY) specifications: Amendment 5 (#933,6171): Enhancements for higher throughput, May 2008”, with a MIMO configuration having four space-division streams making it possible to achieve a data rate of 540 megabits per second (Mbit/s) (Appendix 1, paragraph 4.1 of the IEEE document) for the data field (PSDU).
A SISO type UWB-OFDM system defined in the Wi-Media/ECMA-368 standard makes it possible to achieve a data rate lying in the range 53 Mbit/s to 480 Mbit/s.
A UWB-OFDM system at 60 GHz (e.g. a system derived from the ECMA-368 standard transferred to 60 GHz associated with digital modulation having a larger number of states and with a broader transmission band, as described in the document “Deliverable D2.5, October 2009” accessible at the Internet address http://www.ict-omega.eu/publications/deliverables.html of the European ICT Omega project), or for example the UWB-OFDM system described by the authors I. Siaud and A. M. Ulmer-Moll in the article “Harmonized multi-RF band UWB-OFDM air interfaces for WPAN applications”, MGWS'09 Workshop, Tokyo, September 2009, or indeed a system of the IEEE 802.15.3c standard defined in the IEEE P802.15.3c/D07 document “Part 15.3: Wireless medium access control (MAC) and physical layer (PHY) specifications for high rate wireless personal area networks (WPANs): Amendment 2: Millimeter-wave based alternative physical layer extension”, May 2009, makes it possible to achieve a data rate of several Gbit/s, depending on the channel selected.
Depending on the radiated transmission power level, the radio coverages of the 802.11n system are close to those of WPAN systems (short-range network: distance d (range)<20 meters (m)) using a UWB-OFDM transmission technique.
In order to establish communication between two communicating entities complying with the first example and spaced apart by a distance d, while guaranteeing a data rate D and a target binary error rate BERt representative of a given quality of service QoS, the problem arises of knowing what criterion to use for selecting a transmission mode.
Selection by data rate discrimination (or data rate selection) is described in the documents prepared in the context of a European project known as MAGNET (IST-FP6) accessible at the Internet address: http//magnet.aau.dk/. That selection acts between two transmission modes, one transmission mode associated with an interface I1 of the UWB type delivering data rates of about 250 kilobits per second (kbit/s) (UWB-FM technique), and the other transmission mode, associated with an interface I2 of the multi-carrier type with spreading in the frequency domain by means of codes (multi-carrier spread spectrum (MC-SS) technique) delivering data rates lying in the range 1.8 Mbit/s to 130 Mbit/s operating at 5.2 GHz, implemented in a given communications entity Pnc, Dev.
That transmission mode selection is managed in the IST-FP6 MAGNET system at the level of a layer common to both transmission modes and referred to as the universal convergence layer (UCL). FIG. 3 is a diagram of the makeup of an IST-FP6 MAGNET system. The convergence layer UCL is situated immediately above the protocol layer (MAC) of each of the transmission interfaces I1 and I2, and it generates the messages of exchanges between a first communications entity, a transmitter Pnc, and a plurality of second communications entities, receivers Dev, in order to establish communications with a given interface.
A communications entity may equally well be a transmitter and/or a receiver.
Such selection by data rate discrimination is appropriate when the data rates are very different, however when the transmission modes deliver data rates that may be equivalent, such selection is not satisfactory.
By way of example, mention may be made of a communications entity comprising transmission interfaces that are compatible respectively with a UWB-WiMedia (ECMA-368) system, with an MC-SS system of the MAGNET European project, and with an IEEE802.11n system, all of which cover common ranges of data rates for a given distance d and a given WLAN/WPAN deployment scenario (where a wireless local area network (WLAN) has a distance d (range) close to 150 m in open space, while a wireless personal area network (WPAN) has a distance d (range) close to 15 m to 20 m). A propagation scenario is usually described by a radio coverage range that is typically less than 20 m for the WPAN scenario, a multiple-path channel model, and a corresponding attenuation model. These various models may be different depending on the transmission bandwidth of the transmission mode under consideration.
The MC-SS system of the MAGNET European project, shown in FIG. 4, relies on a spread spectrum transmission technique applied in the frequency domain followed by OFDM modulation. The data rate depends on modulating data symbols (digital modulation or signal binary coding) and on the error correcting coding rate, on the number of spreading codes implemented, and also on the transmission bandwidth (20 megahertz (MHz) or 40 MHz channels), i.e. on the number of data symbols transmitted simultaneously by the OFDM modulation implemented downstream from the spreading process. The system transmits at a carrier frequency equal to 2.4 GHz or 5.2 GHz. FIG. 5 illustrates the various data rates D that can be obtained for different transmission modes associated with a transmission interface of the MC-SS type for a MAGNET system. The circles and squares in FIG. 5 correspond to the different modes that enable the following to be obtained respectively: either a data rate at 40 Mbit/s to within about 3%, or else a data rate of 80 Mbit/s to within about 3% with an upper limit of about 8%. The data rate D that is obtained is proportional to the load on the system (load equal to the number Nc of spreading codes implemented divided by the size Lc of the spreading code), to the size of the transmission band via the number of data symbols spread per OFDM symbol and the modulation and coding scheme (MCS). Depending on the number Nc of the spreading codes implemented, the data rate is given by:
                    D        =                                                            N                BPSC                            ·              r              ·                              N                SD                                      Tsymb                    ·                      Nc            Lc                                              (        1        )            
where NBPSC is the number of bits per data symbol, NSD is the number of symbols per OFDM symbol dedicated to transmission, Tsymb is the duration of the OFDM symbol, and r is the error correcting coding rate.
The granularity of the data rate for each modulation and coding scheme (MCS) is given by the data rate delivered by the system when only one code is implemented (Nc=1). Table A.1 in Appendix A gives the data rate values D of an MC-SS MAGNET system obtained as a function of the transmission mode for a load Nc/Lc=Nc/8. The data rate values shown in FIG. 5 correspond to the values of Table A.1 for Nc=8 or Nc=4. The transmission modes that deliver a data rate of 40 Mbit/s (±3%) are those that coincide with the line for data rate D=40 Mbit/s, and that are surrounded by a round shape in FIG. 5. The transmission modes that deliver a data rate of 80 Mbit/s (±3%) are those that coincide with the line for data rate D=80 Mbit/s and that are surrounded by a square in FIG. 5.
The Wi-Media/ECMA-368 system shown in FIG. 6 relies on OFDM modulation in a 528 MHz channel associated with a frequency hopping method applied to successive OFDM symbols. This method makes use of a hopping pattern referred to as a time-frequency coder (TFC) as shown in FIG. 6. The TFC method is applied on a “band-group” made up of three adjacent RF channels using a pattern that takes six OFDM symbols into account. Fourteen RF channels are defined in the {3.1-10.6} GHz band. Two redundancy techniques are implemented at data symbol scale and they are designated in the standard by spreading technique.
The frequency spreading (FS) technique consists in duplicating the symbols on either side of zero frequency in the OFDM multiplex using Hermetian symmetry. This generates a real signal at the output from the OFDM modulator.
The time spreading (TS) technique consists in transmitting the same OFDM symbol twice over different channels given by the TFC hopping pattern. This gives rise to pseudo-diversity resulting from the different RF channels used to transmit the same OFDM symbol.
Each of these two spreading techniques divides the data rate by two. When they are combined with each other, they divide the data rate by four.
The types of digital modulation (signal binary coding) under consideration are quadrature phase-shift keying (QPSK) or dual carrier modulation (DCM) in which each symbol is made up of two bits. The data rate D obtained for the various transmission modes of the (Wi-Media) ECMA-368 system is shown in FIG. 7 with the transmission mode being plotted along the abscissa axis and identified by the type of digital modulation, together with the correcting code rate and with the spreading technique (e.g.: QPSK-1/3-TS+FS).
The high throughput (HT) modes of the IEEE802.11n system shown in FIG. 8 rely on a SISO/MIMO-OFDM technique consisting in parallel use of binary transmission over a plurality of transmission streams Flux_T that are sent to distinct transmit antennas. The data for transmission is demultiplexed. When demultiplexing is performed on the code bits, the various transmission streams output by the demultiplexer are referred to as spatial streams written Flux_S. For the IEEE802.11n system, the number of spatial streams Nss varies from one to four. The number of spatial streams and an increase in the transmission band width (20 MHz or 40 MHz) enable the transmission data rate to be increased by using parallel transmission over a plurality of OFDM transmission systems as shown in FIG. 8. Each transmission system comprises OFDM modulation performed with a transmission channel of variable size (20 MHz or 40 MHz), a MIMO technique, and a modulation and coding scheme specific to each spatial stream (Nss) (in general the scheme is identical for all of the spatial streams). The system transmits at a carrier frequency equal to 2.4 GHz or else to 5.2 GHz.
For the IEEE802.11n system of FIG. 8, the data rate D obtained for the various transmission modes, written MCS11n in the document of the standard, is shown in FIG. 9 where transmission mode is plotted along the abscissa axis and identified by the type of digital modulation MOD and the correcting code rate COD (e.g.: QPSK-1/2), with this applying to a Tcp=800 nanoseconds (ns). In this standard, MCS11n transmission modes are numbered as a function of the modulation and coding scheme MCS and as a function of the number Nss of spatial streams implemented.
The 802.11n system may implement so-called space-time coding techniques. These do not modify the data rate, but they give rise to spatial redundancy between the spatial NSS streams, thereby increasing the number of transmission streams output by the block STBC. The streams as formed in this way are referred to as space-time streams.
The values obtained for the data rate D as a function of the transmission mode are given in Table C.1 of Appendix C.
Given that the definition of the transmission mode in the meaning of the invention includes the parameter set for a MIMO technique and for the associated coding (space division multiplexing (SDM), Alamouti space time block coding (STBC), etc.), which means that a given SCM11n of an IEEE802.11n system covers a plurality of transmission modes in the meaning of the invention. For example, a configuration comprising a MIMO technique of the SDM type in which the space streams are not subjected to any space-time coding (Nss=NSTS=NTX) as shown in FIG. 10, and a configuration comprising an STBC technique as shown in FIG. 11 gives rise to data rates that are identical and that are referenced in identical manner and covered by the same SCM11n in the IEEE802.11n standard. In the definition of transmission mode in the meaning of the invention, these two configurations correspond to two distinct transmission modes that are distinguished by the MIMO technique and the associated coding that are implemented.
The three above systems may operate in the same frequency band. The IEEE802.11n system shown in FIG. 8 operates over the 2.4 GHz and 5.2 GHz RF channels. The MC-SS system of the MAGNET European project shown in FIG. 4 uses the RF channels of the IEEE802.11n system. The Wi-Media/ECMA-368 system shown in FIG. 6 transmits in the {3.1-10.6}GHz band.
In order to deliver a data rate D of about 80 Mbit/s, the communications entity that includes transmission interfaces compatible respectively with a UWB-WiMedia (ECMA-368) system, with an MC-SS system of the MAGNET European project, and with a IEEE802.11n system may have their parameters set in different ways:                selecting a transmission mode associated with the Wi-Media/ECMA-368 system with a transmission bandwidth of 528 MHz (507.375 MHz of effective bandwidth) in the {3.1-10.6}GHz band may have the following parameter settings: QPSK 1/2 modulation associated with the time spreading (TS) and frequency spreading (FS) method of the standard (Appendix 1); or        selecting a transmission mode associated with the MAGNET MC-SS system having its parameters set as follows: 16-QAM 3/4 or else 16-QAM 2/3 or indeed 64-QAM 1/2 at full load (Nc/Lc=1) operating at 2.4 GHz or at 5.2 GHz for a 40 MHz band; or        selecting a transmission mode associated with the IEEE802.11n system operating either at 2.4 GHz or at 5.2 GHz with its parameters set as follows: SCM11n{TQPSK-3/4, Nss=2, Bw=40 MHz} or SCM11n{QPSK 3/4, Nss=4, Bw=20 MHz} or 4SCM11n{16-QAM 3/4, Nss=1, Bw=40 MHz} or SCM11n{16-QAM 3/4, Nss=2, Bw=20 MHz}, where Nss corresponds to the number of space streams implemented (FIG. 8).        
It can consequently be seen that selecting the most appropriate transmission mode as a function of a data rate discrimination criterion does not work in this example and does not work in the more general situation where different transmission modes enable the imposed data rate constraint D to be achieved; consequently, data rate cannot be used as a selection criterion under such circumstances.
The power required for delivering a data rate D is known as possibly constituting another selection criterion, in particular for a UNIK® system from the supplier Orange France via the RSSI parameter. The UNIK® system dedicated to voice systems is a system operating with the WiFi technique (Bluetooth, IEEE802.11) and it can switch over to GSM when the received power is below a certain threshold. This threshold is defined by each manufacturer and is referred to as the RSSI parameter. The RSSI parameter is an integer lying in the range 0 to Nmax−1. It corresponds to quantifying the received power between two threshold values with a received power variation margin as specified by each manufacturer. This power is measured at the receiver. In terms of real power value, the RSSI parameter is at least greater than the sensitivity threshold of the system and is accompanied by a greater margin associated with the degradation introduced into the propagation channel by multiple paths.
The mechanisms for switching over between the WiFi technique and the 3G technique are specified in the unlicensed mobile access (UMA) standard defined in the document UMA Architecture (Stage 2) R1.04 (2005-5-2), that is accessible at the address http://www.umatechnology.org/specifications/. The UMA standard has a UMA network interface known as a network controller NC and referenced UNC, which controller manages the switching over and the requests at IP network level in order to allocate a resource to the terminal (MS) when the terminal switches over to the 3G system. While the terminal is in the 3G cell, it switches back to the WiFi technique as soon as it finds itself within range of a WiFi access point and providing authentication is possible and the RSSI criterion is satisfied. In the UMA standard (layer 2) in Appendices A and B, minimum received and transmitted power values are given for each WiFi technique and typical antenna gains in transmission and in reception are also given, for the access point and for the terminal.
This required power criterion is unsatisfactory since its range of variation can be completely different from one system to another, e.g. as a function of the manufacturer of the system, of the technology used by the transmission interface (optical, electromagnetic, wired, etc.), of the frequency bandwidth of the system (UWB, etc.), . . . .
Known mechanisms for selecting a transmission mode that is most suitable for guaranteeing a data rate D, a quality of service QoS, and a range d for a communications entity having a plurality of transmission modes are not satisfactory, given that the various criteria used do not enable selection to be performed under all circumstances, in particular for the above-described communications entities.